The three-dimensional structures of the allosteric enzyme aspartate transcarbamylase will be studied at about 3 A resolution, so that atomic models of its conformers of its interactions with substrates, inhibitors, and allosteric effects can be built. Further study of the 3 A structure of the unliganded enzyme are under way, and studies of its complex with N-phosphonacetyl-L-aspartate and cytidine triphosphate are in progress. Parallel biochemical, and eventual structure studies, are in progress on fragments of the enzyme, on the effects of pH changes, and on chemically and genetically modified enzyme. We also have under way a three-dimensional study of the protein inhibitor of carboxypeptidase A. Theoretical studies by molecular orbital methods are being continued on models of enzyme active sites and on localized molecular orbitals in biochemical important molecules. Further studies of the X-ray diffraction structures of cyclic peptides and other biochemically related molecules are proposed. BIBLIOGRAPHIC REFERENCES: The Crystal Structure of the Mushroom Toxin beta-Amanitin, E.C. Kostansek, W.N. Lipscomb, R.R. Yocum and W.E. Thiessen, J. Am. Chem. Soc. 99, 1273 (1977). Molecular Orbital Studies of Enzyme Activity. IV. Hydrolysis of Peptides by Carboxypeptidase A, S. Scheiner and W.N. Lipscomb, J. Am. Chem. Soc. 99, 3466 (1977).